The present invention relates to a process for the preparation of stable, water-miscible, concentrated solutions of micro-dispersions of 1:2 metal complex azo or azomethine dyes and the obtained concentrated dye solutions/micro-dispersions which are useful in the field of dyeing and printing fibrous materials.
It is known to use metal complex dyes in the form of concentrated stock solutions or as finely ground powders for the preparation of dyebaths or printing pastes useful for dyeing and printing various substrates, such as for example textile fibers, leather, or paper. Numerous methods for preparing stable metal-complex dye solutions or dispersions have been proposed in the pa tent literature, such as for example using organic solvents (U.S. Pat. No. 3,989,452), adding alkaline lithium salts when metallizing non-metallized dyes (U.S. Pat. No. 4,000,965), or membrane separation techniques for desalting and thus stabilizing said dye preparations (U.S. Pat. No. 4,452,608).
Although all these methods have their merits they nevertheless still suffer from shortcomings such as undesirable solvent vapors during preparation and use of said dye preparations, when organic solvents are used; stability and degradation problems when using e.g. lithium salts; time and energy consuming desalting methods; or production of too large amounts of undesired effluents, which need extensive equipment to clean them.
The problem that the present invention is based on, is how to reduce or overcome these shortcomings of the processes for preparing the state-of-the-art dye compositions.
It has now been found that this problem can be solved by providing a new process for the preparation of stable, concentrated preparations of 1:2 metal complex dyes which comprises separating the metal-free dye species from their synthesis slurries followed by a metallization step and the in situ forming of the dye preparations, without isolating the metal complex dyes after metallization.